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Affiliation
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School of Population Health, University of Queensland, Herston, Queensland, Australia. w.hall@sph.uq.edu.au |
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Abstract
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There is increasing evidence that cannabis users who drive while intoxicated put themselves and others at increased risk of motor vehicle crashes. Cannabis produces dose‐related cognitive and behavioural impairments in laboratory and simulator studies [1-3]; cannabis users in surveys are more likely to report being involved in accidents than drivers who do not use the drug (e.g. [4, 5]), and cannabis is the illicit drug detected most often in drivers who have been killed in motor vehicle crashes (see [6] for a review).
Older studies that measured inactive metabolites of cannabis did not show whether drivers were impaired at the time of the accident [6, 7]. More recent studies have provided better evidence that cannabis‐affected drivers are at a higher risk of being involved in crashes. Gerberich et al. [8] found that current cannabis users had a higher rate of hospitalization for accidental injury in a cohort of 64 657 patients from a health maintenance organization (HMO) (RR = 1.96). Mura et al. [9] found a similar relationship in a case–control study of Δ‐9‐tetrahydrocannabinol (THC) in the serum of 900 people hospitalized for injuries in motor vehicle accidents and 900 controls of the same age and sex admitted to the same French hospitals for reasons other than trauma (OR = 2.5). Drummer et al. [10] found an increased culpability in cannabis users (OR = 2.5) in 1420 Australian drivers killed in accidents and a dose–response relationship between blood THC level and culpability.
Cannabis use appears to increase the risk of motor vehicle crashes by two to three times [1], a much lower risk than alcohol (from six to 15 times). Given the lower risk and lower prevalence of cannabis than alcohol use, the proportion of accidents attributable to cannabis is much lower than that for alcohol (an estimated 2.5% of fatal accidents in France compared to 29% for alcohol [11]).
Is there now sufficient evidence to discourage cannabis users from driving by conducting roadside drug testing? Any such policy requires specification of a level of THC in blood that provides per se evidence of impaired driving. Grotenhermen and colleagues, in this issue [12], have derived a provisional definition of a per se level using epidemiological evidence and a meta‐analysis of laboratory and simulator studies. Sensibly, they have erred in the direction of setting a high level that may require downward adjustment in the light of further research ...
Keywords: Cannabis impaired driving
Language: en |